Check the winterization valves on the pipes leading to the back of the water heater. The top and bottom valves should be open and the vertical "bypass" valve should be closed.
2013 Jayco JayFlight 26RKS
2013 Ford F150 Ecoboost FX4
2003 Coleman Bayside / 2011 Jayco Jayfeather X20E -- RIP
2012 Tacoma Sport DCSB / 2013 Tundra SR5 - RIP